How do I avoid this in the future?

I'm know nothing about bike maintenance aside from cleaning and lubing the chain, so I took the commuter in for a service. Got hit with this whopping £300 bill. I had a quick checkup service around November so I assumed it would be alright, apparently not 😅

What can I do to prevent this happening again later on? Any simple tips or ELI5 for an idiot like myself?